The goals of this research project are to apply electrochemical techniques to the study of the Co corrinoid compounds in vitro in order to understand the activation of the Co-C bond in enzymatic reactions. One of our long term objectives has been the study of the redox chemistry of vitamin B12 species with a view toward understanding their involvement in biochemical processes. With this goal in mind, we plan to study two mechanistic schemes: (i) the reductive cleavage of organo-B12 species and (ii) B12 catalyzed reduction reactions. The first project, (i) will be a study of the reduction process of alkylcobalamins, alkylcobinamides, and model systems using electrochemical techniques in order to understand in detail the reductive cleavage mechanism in which the carbon-cobalt bond is cleaved by an electron transfer reaction. Our aim is to obtain information on how substituent group structure affects (a) the reduction potentials of organo-B12 species and (b) the rate constants for electron transfer and for Co-C bond fission. These results could help in understanding how the Co-C bond is activated in enzymatic reactions. In most cases, Co-C bond breaking is exceptional fast following an electron transfer event and electrochemical techniques for following such reactions in the musecond and nanosecond time domain have been developed to study these processes. Experiments to investigate the solvent, pH, and temperature dependence of the reaction steps are also necessary for a complete picture of the mechanisms. The second type electrochemical process to be investigated, (ii), is vitamin B12 catalyzed reductions, e.g., oxygen reduction for aerobic systems. This reaction requires the formation of a reduced vitamin B12 species, the Co(II), B12r, or the Co(I), B12s, which then can react rapidly with oxygen or other substrates. Here the rate and products of the process will be studied by voltammetry and spectroelectrochemistry at various electrodes. We also plan to use surface enhanced Raman spectroscopy (SERS) to elucidate the nature of B12 intermediates in these reactions.